Sigma factor
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A sigma factor is a protein needed for initiation of transcription in bacteria. It is a bacterial transcription initiation factor that enables specific binding of RNA polymerase (RNAP) to gene promoters. It is homologous to archaeal transcription factor B and to eukaryotic factor TFIIB. The specific sigma factor used to initiate transcription of a given gene will vary, depending on the gene and on the environmental signals needed to initiate transcription of that gene. Selection of promoters by RNA polymerase is dependent on the sigma factor that associates with it. They are also found in plant chloroplasts as a part of the bacteria-like plastid-encoded polymerase (PEP).
Crystal structure of Thermus aquaticus RNA polymerase sigma subunit fragment containing regions 1.2 to 3.1
Crystal structure of a sigma70 subunit fragment from Escherichia coli RNA polymerase
Solution structure of sigma70 region 3 from Thermotoga maritima
Solution structure of sigma70 region 4 from Thermotoga maritima
RNA polymerase
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In molecular biology, RNA polymerase, or more specifically DNA-directed/dependent RNA polymerase (DdRP), is an enzyme that catalyzes the chemical reactions that synthesize RNA from a DNA template.
RNA polymerase hetero27mer, human
An electron-micrograph of DNA strands decorated by hundreds of RNAP molecules too small to be resolved. Each RNAP is transcribing an RNA strand, which can be seen branching off from the DNA. "Begin" indicates the 3′ end of the DNA, where RNAP initiates transcription; "End" indicates the 5′ end, where the longer RNA molecules are completely transcribed.
RNAP from T. aquaticus pictured during elongation. Portions of the enzyme were made transparent so as to make the path of RNA and DNA more clear. The magnesium ion (yellow) is located at the enzyme active site.
T7 RNA polymerase producing a mRNA (green) from a DNA template. The protein is shown as a purple ribbon (PDB: 1MSW)